The known composite forming fabrics comprise two essentially separate woven structures, each of which includes its own sets of warps and wefts, and each of which is woven to a pattern selected to optimise the properties of the layer. The paper side layer provides, amongst other things, a minimum of fabric wire mark to, and adequate drainage of liquid from, the incipient paper web. The machine side layer should be tough and durable, provide a measure of dimensional stability to the forming fabric so as to minimize fabric stretching and narrowing, and be sufficiently stiff to minimize curling at the fabric edges. Numerous fabrics of this type have been described, and are in industrial use.
The two layers of the known composite forming fabrics are interconnected by means of either additional binder yarns, or intrinsic binder yarns. Additional binder yarns serve mainly to bind the two layers together; intrinsic binder yarns both contribute to the structure of the paper side layer and also serve to bind together the paper and machine side layers of the composite forming fabric. The paths of the binder yarns are arranged so that the selected yarns pass through both layers of the fabric, thereby interconnecting them into a single composite fabric.
In these known composite fabrics, additional binder yarns were generally preferred over intrinsic weft binder yarns, as they were believed to cause fewer discontinuities in the paper side surface of the composite fabric. Recently, both single and paired intrinsic warp or weft binder yarn arrangements have been proposed. However, intrinsic weft binder yarns have been found to cause variations in the cross-machine direction mesh uniformity. Composite fabrics in which intrinsic weft binder yarns are incorporated have been found to be susceptible to lateral contraction under the tensile load placed upon them in a papermaking machine. These intrinsic weft binder yarns have also been found to be susceptible to internal and external abrasion, leading to catastrophic delamination of the composite fabric. Further, due to the necessity of having to weave into the fabric structure additional weft yarns to form the paper side layer, and to bind the paper side layer and machine side layer these fabrics are expensive to produce. More recently it has been proposed to use intrinsic warp binder yarns in pairs, so as to overcome at least some of these disadvantages. The use of pairs offers the advantages that the two warp binder yarns can be incorporated in sequence in successive segments of an unbroken warp path in the paper side surface, and that there is more flexibility of choice for the locations at which each member of the pair interlaces with the machine side layer wefts. It is thus possible to optimise the paper side surface to some extent, for example by reducing marking of the incipient paper web, and to improve the machine side layer wear resistance of the fabric, essentially by increasing the amount of material available to be abraded away before catastrophic failure, usually by delamination, occurs. In these fabrics using pairs of warp binder yarns, each of the paper side layer and machine side layer have separate warp yarn systems, one of which completes the paper side layer weave, and the other of which completes the machine side layer weave.
In the following discussion of this invention, it is to be understood that in a notation such as "2.times.2" the first number indicates the number of sheds required to weave the pattern, and the second number indicates the number of wefts in the pattern repeat. Thus a 2.times.2 pattern requires two sheds, and there are two wefts in the pattern repeat.
It has now been discovered that it is not necessary to provide a separate machine side layer warp system in a warp tied fabric. It is possible to weave a fabric having acceptable paper making properties by utilizing triplets of warp yarns so that each member of the triplets interweaves separately in sequence with the paper side layer wefts, and so that the members of the triplets interlace in pairs with the machine side layer wefts.
Accordingly, the present invention seeks to provide a forming fabric whose construction is intended at least to ameliorate the aforementioned problems of the prior art.
The present invention further seeks to improve upon the known fabrics in which paired warp binder yarns are used. The present invention seeks to provide a forming fabric having reduced susceptibility to cross-machine direction variations in the paper side layer mesh uniformity than comparable fabrics of the prior art. Additionally, this invention seeks to provide a forming fabric that is resistant to lateral contraction.
This invention also seeks to provide a forming fabric that is more efficient to weave than comparable fabrics utilizing intrinsic weft binder yarns to interconnect essentially separate paper and machine side layer woven structures. This efficiency is further enhanced in some of the preferred embodiments, because it is now possible to weave some of the preferred embodiments of the fabric from a single warp beam, because all of the warp yarns follow essentially similar paths, which have equal path lengths within the weave structure.
Furthermore, this invention seeks to provide a forming fabric that is less susceptible to dimpling of the paper side surface.
In a preferred embodiment, this invention seeks to provide a forming fabric having a lower void volume than a comparable forming fabric utilizing intrinsic weft binder yarns.
This invention additionally seeks to provide a forming fabric that is resistant to delamination.